A conventional wheel unit used in a conventional baby carriage, as shown in FIGS. 4-6, comprises two wheels 4 and 4' fixed by a shaft support 5 pivotally connected to a support rod 6. The shaft support 5 supports a shaft 7, which passes through the wheel 4' and then through the wheel 4 before being connected to a fixer 72. The fixer 72 has an annular groove 71 which receives a pinch ring 73 for preventing the wheels 4 and 4' from falling off. The wheel unit is covered by a seal cover 74. When the pinch ring 73 is pulled downwardly, it disengages from an annular groove 71, thereby allowing the wheel 4 to be removed from the shaft 7. The conventional wheel unit shown in FIGS. 4-6 has the following disadvantages:
1. The structure of the wheel unit is too complicated.
2. The pinch ring 73 and the shaft 7 are both made of metal, which is susceptible to rust.
3. Wheels 4 and 4' may have different central opening sizes, which increases the manufacturing costs.